Most steels, ferritic steels and austentic steels are difficult to nitride or carburize. Thermochemical treatment, like carburization and/or nitriding, affects the dimensional stability of components, which require further machining, limit the choice of metallic materials as well as represent a time consuming, additional steps due to the need for heating-up the entire part/component to be treated with further heat treatment.
It is well known in the art that laser can be used as a controlled heat source for hardening the surface of a metallic component. When laser is traversed onto a steel surface, a very thin superficial surface layer heats up very fast to the thermodynamic stability region of austenite and then quenches down (rapid cooling or autoquenching) due to the caloric mass of the material volume of the metallic component, which results in the formation of martensite. This martensite is untempered and tends to a very fine to amorphous microstructure.
Another prior art technique which uses a laser heat source represents “laser nitriding” which was, inter alia developed in the nineties for a local nitridation of cylinder liners in grey cast iron by using a UV-Excimer-Laser (λ=308 nm). This prior art process and the resulting surface layer produced on cast irons are disclosed, for example, in the patent applications DE 197 06 833 A1, DE 197 42 739 A1 and DE 10 2006 057 940 A1. For instance, after repeated treatment of this laser nitriding process, approximately 16-18 wt.-% of nitrogen are diluted in the grey cast iron surface in a depth of up to 2 μm. Laser treatments act very locally on the surface of the metallic material to be treated. However, laser treatment requires significant investments in the laser source.
Another known prior art technique for hardening of surfaces is the inductive hardening. In this technique, the whole surface of ferromagnetic steels is heated by a magnetic field. The steel hardnesses increase to approximately 850-1000 HV (Vickers Pyramid Number HV) by amorphisation and/or nanosizing. This technology is frequently applied to gears. The penetration depth can be reduced by applying a dual frequency magnetic field at high frequencies, as disclosed by FR 2 790 007, using very high powers for a short time. The generation of a magnetic field, however, only allows the treatment of specific geometries. Moreover, electro-magnetic hardening requires large investments in high-frequency, high power magnetic field generators.
Further, electrical discharge surface treatment is known from US 2011/0135845 wherein a coating on a treated metal is intentionally created by melting an electrode material by the energy of a pulsed electrical discharge (surface alloying technique).
It is an object of the present invention to overcome or ameliorate the obstacles of the prior art methods. In particular, it is an object to provide a method and device which allows for preferably easy and reliable surface treatment of a workpiece, preferably locally applied on tribologically stressed surfaces of metallic materials.
It is a further object of the invention to provide a device and method for hardening the surface of a metallic workpiece. It is a further object of the invention to provide a device and method for densification of a treated surface. It is yet a further object of the invention to provide a device and method for surface modifications with minerals to generate predefined tribo-mechanical and/or tribo-corrosive properties.
The objects of the invention are accomplished by the independent claims. Further preferred embodiments are defined by the dependent claims.